Explanation of volt and ampere rating on a product or a source

i don't have a clear understanding of both volt and ampere rating on a product or a source

why ampere is also rated in a product or a source

for example a source 12v and capable of 500mA
and a product rated as 12v and 300mA

doesn't it mean the resistance of the product is r = v/i = 12v/300mA

so when connected to the source(12v) it will automatically draws i = v(source)/r(product) = 12v /(12v/300mA)= 300mA and wouldn't draw 500mA

so why ampere is rated in the product, only voltage would have done the purpose, in this case just a 12v source. As a 12v source would have provided 300mA.

OR why both voltage and ampere are rated only one would have solved the purpose

ampere 2a 500ma maximum current

It is important to know what the source is capable of.

If the source is capable of 200mA, you can't connect a product that needs 400mA.

does voltage and ampere has to be same

rajeshmarndi said:

i don't have a clear understanding of both volt and ampere rating on a product or a source

why ampere is also rated in a product or a source

for example a source 12v and capable of 500mA
and a product rated as 12v and 300mA

doesn't it mean the resistance of the product is r = v/i = 12v/300mA

so when connected to the source(12v) it will automatically draws i = v(source)/r(product) = 12v /(12v/300mA)= 300mA and wouldn't draw 500mA

so why ampere is rated in the product, only voltage would have done the purpose, in this case just a 12v source. As a 12v source would have provided 300mA.

OR why both voltage and ampere are rated only one would have solved the purpose

Click to expand...

A voltage source does not have a constant resistance.
Rating current of voltage source is a supply current value when the output voltage reduses in some percentage, normally say 10%.

For a consumption product, resistance is function of supply voltage (not linear )
Rating current is average value and make a consumption power. Immediate current may much different from rating one especially in power-on stage.

what does 12v 1amp mean

if a source is capable of 500mA and the product requirement is 200mA. Does that mean the source is able to provide 200mA and safe to use OR both ampere and volt has to be same

20 volt 200ma =13v @

so when connected to the source(12v) it will automatically draws i = v(source)/r(product) = 12v /(12v/300mA)= 300mA and wouldn't draw 500mA

Click to expand...

Yes.

so why ampere is rated in the product, only voltage would have done the purpose, in this case just a 12v source. As a 12v source would have provided 300mA.

Click to expand...

Each data serves a purpose. They have different meanings, however:

- Source voltage: The nominal output voltage. In case of a regulated supply, it can be expected almost constant between zero and nominal load current (Source internal resistance equal to zero). An unregulated supply may have a considerably higher open circuit voltage. You shouldn't make assumptions about source internal resistance unless specified.

- Source current: The maximum allowed continuous current. You shouldn't make assumptions about the behaviour in case of overcurrent, unless clearly specified. Some supplies have safe current limit or overcurrent shutdown, others may be permanently damaged by overload.

- Load (consumer) voltage: Typically, a voltage range is specified, otherwise the rated voltage must be expected minimum and maximum voltage as well. Exceeding the maximum voltage can cause device damage.

- Load current: The expectable (maximum) current consumption at nominal supply voltage. The load's voltage-current characteristic may be quite different depending on the load type. As said, it's often non-linear.

As a conclusion: You should achieve Vsource = Vload and Isource >= Iload

12 v = ampere?

In addition to what all is discussed these ratings provide one to select a suitable supply for a given appliance (load / device). This is important in terms of cost and space required and other considerations. As has been pointed out above the open circuit voltage of an unregulated powersupply could be much higher than the rated value. This means when the rated load is connected (say 500 mA), the voltage will be close to the rated value. There are chances that if you select a 12 V 5A unregulated power supply, it will give some 20V DC open circuit (no load / very low load) voltage. If a low current consuming circuit is connected, it may see some 18-20 V DC. If the components in the circuit (which is rated at 12V) have 16V rating may malfunction or fail in long run. This is one technical reason for providing the rating in that fashion.

500ma supply instaed of 300ma

The voltage for the source should closely match to the product.
If the product requires 12V, you should use a 12V power supply.
13V would probably work too, because most
circuit system have their own voltage regulator inside.

Don't rely too much on the text printed on the source device.
Some may indicate 12V 1A on the sticker outside,
but may generate 15V instead.
Some may be 12V, but the current limit is about 0.5A,
after which the voltage drop significantly.

For current rating on the source device,
it indicates how much current it is capable of supplying at the given voltage.
12V 1A, means it can provide the current of 0A to 1A
while maintaining it's 12V. After 1A, the voltage is not guarantee.

For the current rating on the product,
it indicate the maximum current the product will draws from it's power supply.
Usually the current drawn is lesser.
The amount of current drawn depends on the activity
of the product. The specification will tell us that it will not
draw more current beyond what is indicated.

For a 12V 1A device, you might consider a
power supply rating of 12V 2A, with 1A safety margin.
The amount of current in use depends on the product,
and not on the power supply. The power supply
just need to ensure it is capable of supply the request amount
of current that the product needs.

Some reference on power supply,
**broken link removed**


Best Regards,
Siong Boon